Niobium-based alloy that is resistant to aqueous corrosion

Abstract

In various embodiments, a metal alloy resistant to aqueous corrosion consists essentially of or consists of niobium with additions of tungsten, molybdenum, and one or both of ruthenium and palladium.

Claims

1. A piece of chemical-processing equipment shaped to contain an acidic process fluid and composed of a metallic alloy consisting essentially of (i) 1 weight percent—10 weight percent tungsten, (ii) 0.5 weight percent—10 weight percent molybdenum, (iii) at least one of ruthenium or palladium collectively present at 0.2 weight percent—5 weight percent, and (iv) the balance niobium.

2. The equipment of claim 1, wherein a grain size of the metallic alloy is greater than 6 microns.

3. The equipment of claim 1, wherein the alloy comprises both ruthenium and palladium.

4. The equipment of claim 1, wherein the alloy contains 2 weight percent—10 weight percent tungsten.

5. The equipment of claim 1, wherein the alloy contains 2 weight percent—10 weight percent molybdenum.

6. The equipment of claim 1, wherein the alloy contains at least one of ruthenium or palladium collectively present at 2 weight percent—5 weight percent.

7. The equipment of claim 1, wherein the equipment has the form of a plate, a sheet, or a tube.

8. The equipment of claim 1, wherein the equipment comprises a heat exchanger, a lined vessel, a static mixer, or a pump.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which:

(2) FIG. 1 illustrates the conditions for the chemical processing industry that pure niobium will absorb hydrogen and become embrittled when exposed to hot HCl;

(3) FIG. 2 illustrates the conditions for the chemical processing industry that pure niobium will absorb hydrogen and become embrittled when exposed to hot H.sub.2SO.sub.4; and

(4) FIG. 3 is a partial schematic of a heat exchanger in accordance with various embodiments of the invention.

DETAILED DESCRIPTION

(5) As used herein, the singular terms “a” and “the” are synonymous and used interchangeably with “one or more.” Accordingly, for example, reference to “a metal” herein or in the appended claims can refer to a single metal or more than one metal. Additionally, all numerical values, unless otherwise specifically noted, are understood to be modified by the word “about.”

(6) A niobium or niobium-based alloy that is resistant to aqueous corrosion, more particularly to corrosion from acids and resistant to hydrogen embrittlement. The starting niobium is pure or substantially pure. Substantially pure niobium would be a niobium alloy which has up to about 1.1% by weight of non-niobium components.

(7) The niobium or niobium-based alloys may be prepared using a vacuum melting process. Vacuum arc remelting (VAR), electron beam melting (EBM) or plasma arc melting (PAM) are methods of vacuum melting that may also be used for alloying. To formulate the actual alloy, at least one element selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium, platinum, molybdenum, tungsten, and ruthenium (Ru, Rh, Pd, Os, Ir, Pt, Mo, W, and Re) are added to the pure niobium material or substantially pure niobium material or niobium alloy using one of the vacuum melting processes listed above. Although it is noted that VAR, EBM or PAM may all be used, one preferred technique is VAR.

(8) Alternative embodiments of this invention include adding one or more elements other than (or in addition to) the elements listed above that improve the corrosion and hydrogen embrittlement resistance. These additional elements may include yttrium, gold, cerium, praseodymium, neodymium, and/or thorium.

(9) Each of the metals may be present as 10 weight percent or less, 5 weight percent or less, less than 10,000 ppm of the alloy, less than 5,000 ppm of the total amount of the alloy, or even less 2,000 ppm of the total amount of alloy. The metal may be added in an amount of at least 50 ppm, at least 100 ppm, at least 150 ppm, at least 200 ppm, or even at least 250 ppm.

(10) Various embodiments of the invention feature the addition of platinum, as platinum has the greatest number of free electrons to theoretically pull in additional oxygen atoms to close the holes in the Nb.sub.2O.sub.5 oxide layer and/or provide sites of low hydrogen overvoltage thereby stabilizing the Nb.sub.2O.sub.5 oxide layer.

(11) Various embodiments use the addition of ruthenium, rhodium, palladium, osmium, and/or iridium (also known as “platinum group metals,” PGM) which also would provide sites of low hydrogen overvoltage thereby stabilizing the Nb.sub.2O.sub.5 oxide layer.

(12) Still another embodiment uses the addition of molybdenum since it has the same crystal structure, a similar lattice parameter, and complete solid solubility in both niobium and tungsten. This is shown in Table 1 and FIG. 1.

(13) TABLE-US-00001 TABLE I Crystal Structure and lattice Parameters for Refractory Elements Lattice Parameter Element Symbol Crystal Structure (Å) Niobium Nb body centered cubic (bcc) 3.301 Tungsten W body centered cubic (bcc) 3.16 Molybdenum Mo body centered cubic (bcc) 3.15 Platinum Pt face centered cubic (fcc) 3.931 Rhenium Re hexagonal close packed (hcp) a 2.761, c 4.458

(14) Another embodiment uses the addition of rhenium since rhenium has the same crystal structure and a similar lattice parameter to niobium and tungsten.

(15) Niobium ingots formulated using VAR or PAM may then be used to produce plate, sheet, and tube products in a manner similar to that used to manufacture these same products from pure niobium or niobium alloy.

(16) The advantages of the new alloys are superior corrosion and hydrogen embrittlement resistance over pure niobium. Various embodiments feature the addition of platinum, since platinum has the greatest number of free electrons to theoretically pull in additional oxygen atoms and help close the holes in the Nb.sub.2O.sub.5 oxide layer and/or provide sites of low hydrogen overvoltage thereby stabilizing the Nb.sub.2O.sub.5 oxide layer.

(17) As mentioned above, niobium alloys in accordance with embodiments of the present invention may be advantageously utilized to form equipment for the chemical-processing industry. Such equipment may include, for example, heat exchangers, lined vessels, static mixers, and pumps. FIG. 3 depicts an exemplary heat exchanger 300 in accordance with various embodiments of the invention. As shown, heat exchanger 300 features a set of tubes 310 disposed within a shell 320. The tubes 310 are fluidly connected to a process inlet 330 and a process outlet 340. The volume within the shell 320 surrounding the tubes 310 is fluidly connected to an exchange inlet 350 and an exchange outlet 360. During operation, a process fluid flows through the process inlet 330, through the tubes 310, and out the process outlet 340. The process fluid may include, consist essentially of, or consist of an acidic fluid. For example, the process fluid may include, consist essentially of, or consist of one or more of hydrochloric acid, nitric acid, phosphoric acid, sulfuric acid, or acetic acid, or other acids or combinations of acids used in equipment for the chemical processing industry. The process fluid may be at approximately room temperature, or it may be heated (to, e.g., approximately 80° C. to approximately 250° C., or even to approximately the boiling point of one or more components of the fluid). In other embodiments, the process fluid may include, consist essentially of, or consist of one or more aqueous or molten salts.

(18) In order to resist or substantially prevent aqueous corrosion by the process fluid, one or more portions of the heat exchanger 300 may include, consist essentially of, or consist of a niobium alloy in accordance with embodiments of the present invention. The shell 320, exchange inlet 350, and exchange outlet 360 (or, in various embodiments, portions of these components not contacting the process fluid during operation) may include, consist essentially of, or consist of a different material (e.g., steel such as stainless steel). However, the process inlet 330, tubes 310, and process outlet 340 may include, consist essentially of, or consist of a niobium alloy in accordance with embodiments of the present invention in order to resist corrosion by the process fluid. In various embodiments, the entireties of one or more of the process inlet 330, tubes 310, and process outlet 340 include, consist essentially of, or consist of the niobium alloy, while in other embodiments, one or more of these components includes, consists essentially of, or consists of a different material (that may not adequately resist corrosion by the process fluid, e.g., steel such as stainless steel) lined (e.g., at least on the inner surface facing the process fluid) with a layer of the niobium alloy.

(19) During operation, a heat-exchange fluid flows into the exchange inlet 350, through the shell 320 and around the outer surfaces of the tubes 310, and out the exchange outlet 360. The heat-exchange fluid is typically at a temperature different from that of the process fluid, and thus the heat-exchange fluid exchanges heat with the process fluid through the thicknesses of the tubes 310. Thus, the process fluid is either heated or cooled, depending upon the relative temperatures of the process fluid and the heat-exchange fluid. The heat-exchange fluid may include, consist essentially of, or consist of, for example, air, water, steam, and/or any other fluid not corrosive to the shell 320 (or portions thereof not including, consisting essentially of, or consisting of the niobium alloy). In order to promote heat exchange along substantially the entire length of each of the tubes, one or more baffles 370 may be present within the shell 320. The baffles 370 may direct flow of the heat-exchange fluid in a non-straight-line (e.g., sinuous) path between the exchange inlet 350 and the exchange outlet 360 so that the heat-exchange fluid contacts all portions of each tube 310 substantially evenly. (Without the baffles 370, the heat-exchange fluid may flow directly from the exchange inlet 350 to the exchange outlet 360, and one or more portions of the tubes 310 (and the process fluid therein) may not exchange heat with the heat-exchange fluid efficiently.)

(20) In various embodiments of the invention, rather than the process fluid flowing through the tubes 310 and exchanging heat with a heat-exchange fluid flowing through shell 320 and around the tubes 310, the heat-exchange fluid flows through the tubes 310 and the process fluid flows through the shell 320 and around the outer surfaces of the tubes 310. In such embodiments, the portions of the shell 320 and at least the outer surfaces of the tubes 310 may include, consist essentially of, or consist of a niobium alloy in accordance with embodiments of the invention in order to resist corrosion due to the process fluid. The outer surfaces of the tubes 310 and the inner surfaces of the shell 310 (and the outer surfaces of the baffles 370, if present) may be lined with a layer of the niobium alloy, or the entireties of these components may include, consist essentially of, or consist of the niobium alloy.

(21) As detailed above, niobium alloys in accordance with embodiments of the present invention may feature niobium or substantially pure niobium with one or more metal elements present at concentrations up to their solubility limit in the niobium. Various embodiments of the invention feature multiple different metal elements added to the niobium matrix. Such embodiments may advantageously have superior mechanical strength and/or other advantageous properties in addition to resistance to aqueous corrosion. As detailed above, the various alloying elements may be alloyed with the niobium by methods such as LAM, VAR, EBM, or PAM. The elements may be alloyed with the niobium individually (i.e., as serial additions), or more than one (or even all) of the elements may be alloyed with the niobium together at the same time. Once alloyed, the metallic alloys in accordance with embodiments of the invention may subsequently be mechanically worked (e.g., by rolling forging, extrusion, etc.) and annealed to, for example, recrystallize the grain structure of the alloy. Alloys in accordance with embodiments of the invention may be annealed at temperatures of, for example, 1900° F.-2300° F. and may be recrystallized at levels of, e.g., approximately 95% to approximately 100%. The resulting grain size of alloys in accordance with embodiments of the invention may be greater than 6 microns, greater than 7 microns, or even greater than 8 microns. The grain size of alloys in accordance with embodiments of the invention may be less than 20 microns, less than 15 microns, less than 12 microns less than 10 microns, or even less than 9 microns.

(22) Metallic alloys in accordance with exemplary embodiments of the present invention may include, consist essentially of, or consist of niobium alloys containing 1-5 weight percent W, 0.5-5 weight percent Mo, and Ru and/or Pd individually or collectively present at 0.2-5 weight percent. In various embodiments, the W is present at 2-3 weight percent. In various embodiments, the Mo is present at 1-2 weight percent. In various embodiments, the Ru and/or Pd are individually or collectively present at 0.2-2 weight percent, 0.2-1 weight percent, or 0.2-0.5 weight percent. The alloy may contain Ru or Pd, but not both, but in some embodiments both Ru and Pd are present in the alloy.

EXAMPLES

(23) A series of samples was fabricated for corrosion testing and evaluation of mechanical properties. Each sample was fabricated by VAR followed by mechanical rolling and subsequent annealing for recrystallization. Portions of each sample were subjected to two different corrosion tests each lasting 15 weeks. The first corrosion test involved submersion in 70% nitric acid (HNO.sub.3) at 150° C. and the second corrosion test involved submersion in 80% sulfuric acid (H.sub.2SO.sub.1) at 14° C. The various samples evaluated are summarized in the table below, where all alloy concentrations are provided as weight percentages. As shown, three comparative samples (CS) were also prepared for comparison to the alloys in accordance with embodiments of the present invention. Comparative Sample 2, i.e., Zr 702, is nominally pure Zr that includes up to 4.5% Hf.

(24) TABLE-US-00002 Sample # Composition 1 Nb—3% W—1.5% Mo 2 Nb—3% W—1.5% Mo—0.2% Ru 3 Nb—3% W—1.5% Mo—0.5% Ru 4 Nb—3% W—1.5% Mo—1% Ru 5 Nb—3% W—1.5% Mo—0.5% Pd 6 Nb—3% W—1.5% Mo—1% Pd CS1 Pure Nb CS2 Zr 702 CS3 Ta—3% W

(25) The results of the corrosion tests are summarized in the table below. The corrosion rates for each of the corrosion tests are provided in mils per year, and the concentration of hydrogen absorbed into each sample during each test is also reported. A negative corrosion rate indicates that the weight of the sample increased during the test, perhaps due to reaction with the corrosive agent (e.g., formation of an oxide layer or other byproduct on the sample).

(26) TABLE-US-00003 Corrosion Test #1 Corrosion Test #2 Corrosion Rate H.sub.2 conc. Corrosion Rate H.sub.2 conc. Sample # (mpy) (ppm) (mpy) (ppm) 1 <0.01 2 −31 285 2 <0.01 2 −34 143 3 0.03 2 −24 118 4 <0.01 4 −21 41 5 0.01 5 −24 8 6 0.01 3 −24 29 CS1 0.02 3 Dissolved in less N/A than 15 weeks CS2 −5.2 2 Dissolved in less N/A than 7 weeks

(27) As shown, all of the samples performed comparably to Comparative Sample 1 and Comparative Sample 2 during the first corrosion test. The comparative samples did not survive the second corrosion test and were clearly outperformed by the samples in accordance with embodiments of the present invention.

(28) Various mechanical properties of the samples were also evaluated via tensile testing, and the results are summarized in the table below.

(29) TABLE-US-00004 Ultimate Anneal Grain % Yield Tensile % Sample Temperature Size Recrystal- Strength Strength Elon- # (° F.) (μm) lization (ksi) (ksi) gation 1 1950 7.0 95 29.7 46.3 30.6 2 1950 8.6 98 42.8 52.3 27.0 3 2200 8.6 100 43.2 54.2 17.3 4 2200 8.4 100 43.1 55.8 18.9 5 2200 8.9 100 37.4 49.3 17.0 6 2200 8.6 100 40.6 54.1 25.4 CS1 1800 6.0 100 19.2 31.8 31.3 CS3 2550 6.9 100 38.6 53.8 35.5

(30) As shown, the various samples in accordance with embodiments of the present invention displayed mechanical properties superior to those of pure Nb (Comparative Sample 1) and comparable to those of the Ta-based Comparative Sample 3 but are processable at lower (and thus more economical) annealing temperatures.

(31) All the references described above are incorporated by reference in its entirety for all useful purposes. The terms and expressions employed herein are used as terms and expressions of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof. In addition, having described certain embodiments of the invention, it will be apparent to those of ordinary skill in the art that other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit and scope of the invention. Accordingly, the described embodiments are to be considered in all respects as only illustrative and not restrictive.